This invention relates to a piezoelectric element and to a process for preparing same.
Piezoelectric elements including a piezoelectric layer interposed between a pair of electrodes are now used in a variety of fields. One known piezoelectric layer consists of an inorganic piezoelectric material such as barium titanate or lead titanate zirconate. In spite of its high piezoelectric constant, a piezoelectric element using this type of piezoelectric layer cannot be applied to various utilizations because of the brittleness of the layer. A piezoelectric layer using an organic piezoelectric material such as poly(vinylidene fluoride), poly(vinyl fluoride), poly(vinylidene chloride), poly(vinyl chloride) or nylon is also known. While this layer can be shaped in various desired forms such as films because of its flexibility, the piezoelectric constant thereof is lower than that of the above-mentioned piezoelectric layer of an inorganic piezoelectric material.
A composite piezoelectric layer composed of an organic matrix in which an inorganic piezoelectric material is dispersed is known. As the matrix, the above-mentioned organic piezoelectric materials or other synthetic polymeric materials such as fluoro-rubbers, chloroprene rubbers, silicone rubbers, fluorocarbon resins and epoxy resins are generally used. Since such a composite piezoelectric layer has both a high piezoelectric constant and a high flexibility, piezoelectric element using the composite layer is applied to a wide variety of utilizations such as speakers, buzzers, microphones, key boards, pressure-sensitive switches and flaw detectors. Further, since the composite piezoelectric layer has an acoustic impedance as low as a liquid or a living body, piezoelectric elements using such a layer are applicable to hydrophones or ultrasonic detecting probes for living bodies.
Known piezoelectric elements have been produced by impressing a direct current voltage between a pair of electrodes between which a matrix layer containing an inorganic piezoelectric material is interposed, so that the inorganic piezoelectric material is polarized in one direction.
For increasing piezoelectric constants d.sub.31, d.sub.33 and d.sub.h of the piezoelectric elements, various attempts have been made such as by using a specific piezoelectric material such as of a lead titanate zirconate-series, by increasing the content of the piezoelectric material in the matrix and by increasing the impression voltage for polarization of the piezoelectric material.
In the conventional piezoelectric elements using the above composite piezoelectric layer, when the strain in the direction of d.sub.33 (strain in the direction parallel with the electrical field) is increased, the strain in the direction d.sub.31 (strain in the direction perpendicular to the electrical field) is also increased as a result of a lateral-longitudinal coupling. This is advantageous for some sort of piezoelectric elements but disadvantageous for unidirectivity ultrasonic detecting probes in which ultrasonic waves are emanated or received in only one direction, for hydrophones in which the figure-of-merit is represented by underwater wave receiving sensitivity d.sub.h (=d.sub.33 +2d.sub.31) or for devices which are towed by marine vessels and in which noises are generated during tow.
More particularly, when a composite piezoelectric sheet containing an epoxy resin matrix and lead titanate zirconate powder dispersed in the matrix with a volume ratio of the powder to the matrix of 60:40 is sandwiched between a pair of electrodes and when a direct current voltage of 70 KV/cm is impressed between the electrodes, the resulting piezoelectric element has d.sub.33 of 104.times.10.sup.-12 C/N and d.sub.31 of -45.5.times.10.sup.-12 C/N. When this piezoelectric element is used in air, the wave receiving sensitivity is represented by d.sub.33. When the element used under water such as in the case of a hydrophone, a static water pressure is applied in all directions so that the wave-receiving sensitivity is represented by d.sub.h (=d.sub.33 +2d.sub.31). Namely, the sensitivity d.sub.h is 13.times.10.sup.-12 C/N which is about one eighth of d.sub.33. Thus, in the case of hydrophones, d.sub.31 poses a problem of reduction of d.sub.h due to the negative contribution thereof. Similarly, when such a piezoelectric element is used for towing by a vessel, noise signals are unavoidably generated due to the strain in the direction of d3.sub.1. These signals are amplified by positive signals caused by the strains in the direction of d.sub.33 so that the S/N ratio is lowered.
Further, when the above piezoelectric element is used as an ultrasonic detector probe, in which a high unidirectivity in the direction perpendicular (d.sub.33 direction) to or parallel (d.sub.31 direction) with the electrodes is desired in generating or receiving ultrasonic wave in either d.sub.31 or d.sub.33 direction, d.sub.31 and d.sub.33 of the piezoelectric element are not compatible with each other with respect to the unidirectivity.